Short metal fibers

ABSTRACT

The present invention relates to short metal fibers. A set of short metal fibers, with an equivalent diameter ranging from 1 to 150 $(m)m, comprises entangled and curved fibers. At least 10% of the short metal fibers are entangled, whereas the length of the curved fibers is distributed according to a gamma-distribution, having an average length preferably between 10 and 2000 $(m)m.

FIELD OF THE INVENTION

The invention relates to short metal fibers and to a sintered metalfiber product, using such fibers. The invention further relates to amethod to produce short metal fibers and a sintered metal fiber product,using such fibers.

BACKGROUND OF THE INVENTION

Metal fibers having a rather flat cross section, with diameter less than15 μm and a length of less than 400 μm are known from U.S. Pat. No.4,703,898. These fibers have a crescent shape and have a small,point-like hook at both ends. This document further provides a method toproduce such fibers.

JP2175803 describes similar short metal fibers, which have a curvedshape.

Short metal fibers are also known from GB889583. These metal fibers maybe undulated or “kinked” over their length. In this document, theseterms mean that the major axis of the fibers change two or more timesover the length of the fiber.

SUMMARY OF THE INVENTION

Most known short metal fibers are rather difficult to pour homogeneouslyin a mould, providing identical properties throughout the whole mouldvolume such as density and sizes of open space between adjacent fibers,although lots of improvements have been suggested.

The present invention relates to an alternative set of short metalfibers, which have an improved pourability. Further, the inventionrelates to sintered short metal fiber products and to a method toprovide short metal fibers and sintered short metal fiber products.

A set of short metal fibers used to provide the temperature resistantmaterial as subject of the invention is characterized by the presence oftwo different groups of short metal fibers, being “entangled” fibers and“curved” fibers.

A set of short metal fibers as subject of the invention comprises shortmetal fibers with an equivalent diameter “D” between 1 and 150 μmpreferably between 2 and 100. Most preferably the equivalent diameterranges between 2 and 50 μm or even between 2 and 35 μm such as 2, 4,6.5, 8, 12 or 22 μm.

With the term “equivalent diameter” is meant the diameter of animaginary circle, which has the same surface as the surface of a fiber,cut perpendicular to the major axis of the fiber.

The set of short metal fibers comprises entangled fibers. The number ofentangled fibers in a set of short metal fibers as subject of theinvention ranges from 5 to 35%. Preferably more than 10% of all shortmetal fibers in the set of short metal fibers are entangled. Thesefibers are hereafter referred to as “entangled fibers”. To have astatistically reliable percentage, a sample of at least 50 fibers,randomly chosen out of the set of short metal fibers are to beevaluated.

The percentage of entangled fibers is measured and calculated as:% entangled fibers=100×(#entangled/#total)wherein

#entangled=number of entangled fibers out of the sample;

#total=number of fibers out of the sample.

The entangled fibers of the set of short metal fibers as subject of theinvention have an average length “Le”, which is considerably longer asthe average length of the curved fibers “Lc”. The average length of theentangled fibers is at least 5 times the average length of the curvedfibers. Preferably, the average length of the entangled fibers is morethan 10 times the average length of the curved fibers. Preferably, theaverage length of the entangled fibers is larger than 200 μm, or evenmore than 300 μm, most preferably more than 1000 μm. the entangledfibers may be entangled with themselves (individually) or may beentangled together with some other entangled fibers. The entangledfibers, either individually or together with other entangled fibers,cannot be individualized as an essentially straight fiber out of theshape which is defined by the entanglement of the fibers. The major axisof each fiber changes so often and unpredictably, that the fiber may beentangled in many different ways. Some of the fibers are present in ashape, which resembles to a clew. The effect is comparable to theso-called pilling effect, well known in the textile industry, and incarpet industry more in particular. One or more fibers get trapped intoa small ball. The trapped fibers may not be separated from this ballanymore. Other fibers look more like a pigtail. The are characterized bya major axis which changes several times in an unpredictable way, so arelatively chaotic shape may be provided.

The other short metal fibers out of the set of short metal fibers arehereafter referred to as “curved” fibers

The average length “Lc” of the curved fibers of the set of short metalfibers may range from 10 to 2000 μm, preferably from 30 to 1000 μm suchas 100μm, 200 μm or 300 μm. When a length distribution is measured fromthese curved fibers as part of a set of short metal fibers as subject ofthe invention, a gamma-distribution is obtained. This gamma-distributionis identified by an average length Lc and a shape factor “S”. Accordingto the present invention, the gamma-distribution of the length of thecurved fibers, has a shape factor S ranging between 1 and 10.

For average lengths Lc larger than 1000 μm, usually a shape factor Slager than 5 is measured. For average lengths Lc between 300 μm and 1000μm, a shape factor S between 2 and 6 is usually measured. For averagelengths Lc smaller than 300 μm, usually a shape factor S smaller than 3is measured. To have a statistically reliable distribution, at least 50curved fibers, randomly chosen out of the set of short metal fibers areto be measured.

The L/D ratio of a set of short metal fibers as subject of the inventionhas an L/D-ratio of more than 5, preferably more than 10, wherein L isthe average length of all fibers, present in a representative sample offibers from the set of short metal fibers. As described above, thissample comprises at least 50 fibers out of the set of short metalfibers. Preferably, but not necessarily, the curved fibers out of a setof short metal fibers as subject of the invention has an Lc/D-ratio ofmore than 5, preferably more than 10.

Further, a majority of these curved fibers have a major axis, whichchanges over an angle of at least 90°. This angle is the largest angle,which can be measured between two tangents of this major axis.Preferably, 40% of the curved fibers has a major axis, changing morethan 90°, e.g. more than 45%, or preferably more than 50%. To measurethese curves of the major axis, a microscopic image with appropriateenlargement is taken from several short metal fibers. Using a computerimaging system, the tangents of the major axis and the largest anglebetween them is calculated. To have a statistically reliable sample, atleast 50 curved fibers, randomly chosen out of the set of short metalfibers are to be measured.

Such a set of short metal fibers has several advantages. A set of shortmetal fibers as subject of the invention has good pouring behavior.

Further, when short metal fibers as subject of the invention are poured,e.g. in a specific three-dimensional mould or on a flat surface,numerous contact points can be noticed between the short metal fibers.They are, so to say, ready to be sintered without a major force which isnormally to be applied before sintering. The amount of contact points ispresent without requiring a force, which is not the case when thediameter of the short metal fibers extends beyond 150 μm. Oneunderstands that, if necessary to increase even more the number ofcontact points, or to decrease the pore volume and/or size, such forcesmay be applied before or during further processing.

Once poured, a set of short metal fibers as subject of the invention hasan apparent density in the range of 10 to 40%, according to ISO787-11.The pores between the short metal fibers are very small, but the numberof pores is sufficiently large to provide an apparent density which istypically between 10 and 40%. A porosity, calculated as indicated below,ranges between 60 and 90%.Porosity (%)=100%−apparent density (%)

The volumes between the fibers are similar throughout the poured volume,so providing an isotropic volume.

Short metal fibers as subject of the invention may be obtained by amethod comprising the following steps:

-   -   individualizing the metal fibers by a carding operation;    -   providing the set of short metal fibers by cutting or entangling        and sieving the set of short metal fibers, preferably by using a        comminuting machine.

First, metal fibers, being present in a bundle of fibers, in a yarn or atextile structure, or even as staple fibers, are individualized to someextend by a carding operation.

These more or less individualized fibers are brought into a comminutingdevice. In this device, each fiber is cut into short metal fibers byfast rotating knifes. The blade of these knifes, having a certain bladethickness, encounter or ‘hit’ the fibers usually in radial direction.The fibers are mechanically plastically deformed and entangled orpossibly broken into a smaller length. Due to the centrifugal force, theso provided short metal fibers (curved or entangled) are blown outwardlyagainst the external wall of the comminuting device. This external wallcomprises a sieve with well-defined openings. According to theseopenings, short metal fibers with a certain length may pass through thesieve, whereas too long short metal fibers will stay in the comminutingdevice and possibly be hit once again, until the lengths aresufficiently small to pass the sieve, or until they are entangled enoughto allow passage through the sieve.

The alloy of the metal fibers is to be chosen in order to providerequired properties such as temperature resistance or electricalconductivity. Stainless steel fibers out of AISI 300-type alloys, e.g.AISI 316L or fibers based on INCONEL®-type alloys such as INCONEL®601 orNICROFER®-type alloys such as NICROFER® 5923 (hMo Alloy 59) and NICROFER6023, or fibers based on Fe—Cr—Al alloys may be used. Also Ni-fibers,Ti-fibers, Al-fibers, Cu-fibers or fibers out of Cu-alloy or otheralloys may be used.

Metal fibers may e.g. be bundle drawn or shaved, or provided by anyother process as known in the art.

The short metal fibers as subject of the invention may be used toprovide a sintered product. The method of manufacturing a sinteredproduct comprises the steps of:

-   -   providing a set of short metal fibers;    -   pouring said set of short metal fibers into a three-dimensional        mould or on an essentially plane surface;    -   sintering said set of short metal fibers to form a sintered        product.

Possibly, before the sintering, the short metal fibers are pressedtogether to improve the coherency and/or to change the density.

Alternatively, the short metal fibers are brought in a suspension, usingan appropriate agent or a mixture of appropriate agents.

The suspension is brought into a mould or poured onto an essentiallyplane surface. In a subsequent step the suspension liquid is removed,e.g. evaporated or sucked out. The mould, comprising the short fibers isthen subjected to a sintering process in which all non-metal fiberelements are removed and in which a sintering between the metal fibersis obtained.

The sintering conditions are dependent upon the alloy and the propertiesrequired by the short metal fibers and the final sintered metal fiberproduct.

A great advantage of the method of the present invention is that the setof short metal fibers can easily be poured homogeneously; resulting inisotropic properties over the whole volume of the sinter metal fibersproduct. For example the density and the sizes of the pores arehomogeneous over the whole volume of the product.

Another advantage of sintered products according to the presentinvention is their high porosity.

As indicated above, the short metal fibers will arrange themselvesproviding a three dimensional fiber structure, with numerous small poresand numerous contact points. The pores are characterized by a relativelysmall size.

Applying forces on the short metal fibers during sintering may decreasepore size and porosity.

The porosity of a sintered product according to the invention is equalto the porosity of the poured fibers, as described above. The fibers donot have to be put under pressure to form a coherently sintered volume.

This means that the porosity ranges generally between 60 and 90%.

The porosity is for example 70, 80 or 85%.

However, dependent on the type and level of pressure, the porosity maybe lowered to 49% if necessary, for example by cold isostatic pressing.

According to the specific use of the short metal fibers or the sinteredmetal fiber product, different metals and/or alloys may be used toprovide the short metal fibers or the sintered metal fiber product.

Sintered short metal fiber products may have different shapes, accordingto the specific requirements of their application. Short metal fibersmay be sintered into flat plates, rings, cylindrical or tube likeshapes. Also more complex shapes such as monolithic structures may beobtained.

Sintered products may be used for different applications.

They may be used as a filtering device, for example a filtering deviceto filter gases or liquids.

The alloy of the metal fibers may be chosen in order to provide thefiltering device the required properties such as temperature resistanceand chemical resistance. Consequently, the filtering device may be usedfor high temperature applications, for example for the filtration of hotgases or for the filtration of corrosive gases or liquids.

The filtering device may have any shape. Preferred shapes are flatplates, rings, or cylindrical or tube like shapes.

When a sintered product as subject of the invention is used as afiltering devices, especially after being isostatically pressed, suchfiltering device may have an absolute filter rating of 0.5 μm up to 20μm. Usually, the absolute filter rating may range between ⅓ and ½ of theequivalent diameter of the short metal fibers used.

A filtering device according to the present invention can thus be usedfor microfiltration applications, for example for the filtration of airin clean labs or in product rooms of electronic components.

A sintered product according to the present invention is in particularsuitable to filter diesel exhaust gases.

A product as subject of the present invention may also be used as acarrier for catalysts. Therefore, commercially available catalyst can beapplied on a sintered product.

A sintered product on which a catalyst is applied, hereinafter referredto as the catalyst, can be used to treat exhaust gases such as exhaustgases from incinerators or diesel engines, thereby removing harmfulsubstances, such as NO_(x), NH₃, CO, dioxins, O₃.

The catalyst is characterised by a porous open structure and a highspecific surface. At the same time it is characterised by a highstrength. Since the sintered product may withstand high temperatures,the catalyst can be used at high operating temperatures.

All these features result in a catalyst having a high efficiency ofcatalytic conversion.

Furthermore, a sintered product according to the present invention maybe used as a catalytic filter which combines particle and/or dustretention and catalytic conversion of harmful components.

A sintered product may also be used as heat exchanging device. e.g. inStirling engines, where a sintered product may be mounted in the passageof the working fluid or gas. Such device is also referred to as heatrecuperator. The sintered product is heated when the working fluid orgas passes from the hot to the cold chamber of the Stirling engine.Afterwards, the heat, captured in the sintered product is regeneratedwhen the cold working fluid or gas passes again through the sinteredproduct, while it flows back to the hot chamber of the Stirling engine.

A three-dimensional sintered product according to the present inventioncan also be used a porous mould, for example as a mould to form glassproducts such as windshield glass.

Surprisingly, another use of a set of short metal fibers as subject ofthe invention is found by blending a set of short metal fibers with aceramic matrix or ceramic or high-temperature resistant glue. A blend ofshort metal fibers and ceramic matrix or ceramic or high temperatureresistant glue, up to 15% or even 20% by weight of short metal fibers,seems to resist thermal expansions to a larger extend, compared to thepure ceramic or high temperature resistant glue, once the glue or matrixcomprising short metal fibers are cured. A higher resistance to thermalcracks in the glue was obtained. Preferably, the set of short metalfibers represents at least 0.5% of weight of temperature resistantmaterial. Positive results were obtained especially when a set of shortmetal fibers is used which comprises entangled and curved fibers ofwhich more than 10% of the set of short metal fibers are individuallyentangled fibers.

Surprisingly, only a relatively small change in electrical conductivitywas noticed when the amount of the set of short metal fibers is keptlower than 10% by weight of the temperature resistant material, e.g. inthe range of 1% to 9.5%, in the mean time providing sufficientresistance to thermal shocks and cracks. Higher percentages by weight ofa set of short metal fibers may be used, e.g. more than 15% or even morethan 20% or 30%, however such percentages of weight are not absolutelynecessary to obtain a sufficient resistance to thermal shocks.

This effect is not restricted to short metal fibers as subject of theinvention, but was also noticed using other short metal fibers. Howeverthe presence of entangled fibers plays an important role for theimprovement of the thermal shock resistance. On the other hand, thepresence of the curved fibers provides better pourablity and mixingbehavior into the ceramic matrix or glue.

Preferably, ceramic matrices or ceramic glues based on SiO₂, A1 ₂O₃,ZrO₂ and/or MgO are used.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference tothe accompanying drawings wherein

FIGS. 1A, 1B, 1C, 1D, 1E and 1F are images of short metal fibers, allbeing part of a set of short metal fibers as subject of the invention.

FIG. 2 shows a curved fiber being part of a set of short metal fibers assubject of the invention.

FIG. 3 shows a graph of the length distribution of a set of short metalfibers as subject of the invention.

FIG. 4 shows a graph of the curvature distribution of the curved fibersout of a set of short metal fibers as subject of the invention.

FIG. 5 shows a sintered metal fiber product as subject of the invention.

FIGS. 6 a and 6 b show a sintered product having a monolithic structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

A preferred embodiment of a set of short metal fibers as subject of theinvention is shown in FIGS. 1A, 1B, 1C, 1D, 1E and 1F, which all showshort metal fibers out of the same set of short metal fibers as subjectof the invention. The short metal fibers, having an equivalent diameterof 22 μm, are obtained by providing a bundle of AISI 316L bundle drawnfibers of a carding device and further to a comminuting device. As maybe seen from FIGS. 1A to 1F, the shape of the short metal fibers may bevery different. Some short metal fibers are clearly entangled fibers,such as fibers 11, 12 and 13. Fibers 12 are more curled irregularly,providing a non-defined shape. Fibers 13 are individually entangled to anon-defined shape. Fibers 11, 12 and 13 are to be understood as“entangled fibers”. Other fibers 14 are clearly curved, although thecurling angles are unpredictably. Some fibers, such as fiber 15, mayhave a limited curvature. An example of such a curved fiber is shownschematically in FIG. 2. A curved fiber has two ends, being a first end21 and a second end 22. A major axis 23 connects the center of thetransversal cuts over the whole length of the fiber. The direction ofthe major axis 23 changes over an angle α. Angle α is absolute value ofthe largest angle which can be measured between two vectors 24 having adirection equal to the tangent of the major axis, starting point being apoint of the major axis, and a sense pointing from first end 21 tosecond end 22.

FIG. 3 shows the angle distribution of the change of major axis of thecurved fibers of the set of short metal fibers from FIGS. 1A to 1F. Asample to 316 fibers, randomly chosen out of the total set of shortmetal fibers was taken. Each bar 33 in the graph represents the numberof fibers (to be read at the left ordinate 34), having a major axischanging with an angle α, α being smaller than the angle valueunderneath the bar, which is related to that bar, but larger than theangle, related to the bar at its left side. E.g. the bar related to 90°,indicates the number of curved fibers, having an angle α smaller than90°, but larger than 80°. Related numbers are summarized in Table I

TABLE I % curved with angle α or angel α of number in % curved withangle entangled/total fibers sample α/total curved fibers fibers  0 00.00 0.00  10 2 0.65 0.55  20 3 0.97 0.83  30 10 3.24 2.77  40 16 5.184.43  50 16 5.18 4.43  60 19 6.15 5.26  70 22 7.12 6.09  80 21 6.80 5.82 90 18 5.83 4.99 100 17 5.50 4.71 110 10 3.24 2.77 120 14 4.53 3.88 13014 4.53 3.88 140 15 4.85 4.16 150 28 9.06 7.76 160 18 5.83 4.99 170 3110.03 8.59 180 35 11.33 9.70 entangled 52 — 14.40 total 52 entangledtotal curved 309 total 361

Line 31 indicates the cumulative curve of the number of curved fibershaving an angle α, less than the angle value in abscissa. This number isexpressed, as indicated on the right ordinate 35, in percentage comparedto the total number of curved fibers in the sample. More than 50% of thecurved fibers have a major axis direction changing more than 90°.

As also indicated in FIG. 3, more than 10% of all short metal fibers outof the set of short metal fibers are entangled fibers. This is indicatedby the dots 32, which represent the percentage of fibers, also to beread on the right ordinate 35, comprised in the related bar 33, comparedto the total number of short metal fibers out of the sample taken fromthe set of short metal fibers.

FIG. 4 shows the length distribution of the curved fibers of two sets ofshort metal fibers as subject of the invention.

A first length distribution 41, indicated with black bars, is a lengthdistribution of the curved fibers of a set of short metal fibers, havingan equivalent diameter of 8 μm. The set of short metal fibers wasprovided using bundle drawn stainless steel fibers, alloy AISI 302. Arepresentative and randomly chosen sample of 227 fibers was taken. Anaverage length Lc of 420 μm was found. The length distribution is agamma-distribution 42, being characterized with a shape factor S being3.05. The bars of distribution 41 is to be understood as the percentageof curved fibers out of the sample (read in ordinate 43), which has alength (expressed in μm and indicated in abscissa 44) in the range withupper limit as indicated underneath the bar, and lower limit being thelength indicated under the adjacent bar left if it. In the same way, thegamma-distribution reads the percentage of fibers in ordinate 43 in therange indicated on the abscissa 44 as explained above.

Another length distribution 45 is shown in FIG. 4, indicated with whitebars, which is a length distribution of the curved fibers of a set ofshort metal fibers, having an equivalent diameter of 12 μm. The set ofshort metal fibers was provided using bundle drawn stainless steelfibers, alloy AISI 316L. A representative and randomly chosen sample of242 fibers was taken. This length distribution accords to agamma-distribution 46, which is characterized with a shape factor Sbeing 3,72. An average length Lc of the curved fibers of 572 μm wasmeasured.

A sintered metal fiber product as subject of the invention may beprovided as shown in FIG. 5. The short metal fibers used have a diameterof 22 μm. The thickness 51 of the medium is approximately 40 mm. Thesintered metal fiber product has a porosity of 81%. The short metalfibers were stainless steel bundle drawn fibers, Fecralloy® type alloy.Such ring-like shape may be used as heat regenerating device in aStirling engine. An alternative sintered metal fiber product, as subjectof the invention is obtainable by sintering short metal fibers assubject of the invention to a flat shape, plate-like product. E.g. shortmetal fibers of alloy AISI 444, having an average length of 1000 μm andan equivalent diameter of 65 μm, obtained by a shaping process asexplained in WO9704152, are sintered to a flat volume with thickness of2.35 mm and a weight of 5226 g/m². A porosity of 72% and an absolutefilter rating of 92 μm was obtained. When a similar product, using shortmetal fibers as subject of the invention, being bundle drawn fibers ofan equivalent diameter of 12 μand an average length L of 800 μm,isostatically pressed using 800 Bar, a porosity of 70% was obtained, andan absolute filter rating of 5.3 μm.

One understand that other shapes, such as flat plates, or tube-like orcylindrical shapes may be obtained. Even monolithic structures, e.g. tobe used in a diesel exhaust filter, filtering soot from the exhaust gas,may be obtained.

FIG. 6 a shows a monolithic structure 600 comprising a set of shortmetal fibers. FIG. 6 b shows a cross-section along line A–A′.

The monolithic structure shown in FIG. 6 is in particular suitable tofilter exhaust gases.

The gas to be filtered enters the monolithic structure at inlet side 602and exits the monolithic structure at outlet side 604.

The monolithic structure has a number of cells 606. Each cell has afirst end adjacent the inlet of the monolithic structure and a secondend adjacent the outlet of the monolithic structure.

At least part of the cells is blocked at the second ends of the cell bya barrier 607. The barrier comprises a material that does not allow thepassage of the gaseous stream.

In the preferred embodiment of FIG. 6 a cell is either blocked at itsinlet side as for example cell 608; or at its outlet side as for examplecell 610. Thus, the exhaust stream can not pass freely through thecells, but is obliged to pass through the walls to a neighbouring cellhaving an open outlet side.

For example exhaust gas entering cell 610 passes through the wallsurrounding cell 610, as indicated by the arrows 612, for example tocell 608 through which it can leave the monolithic structure at theoutlet side.

Possibly, the cell walls are coated with a catalyst or alternatively acatalyst is applied into the porous structure of the monolithicstructure.

A three-dimensional sintered product according to the present inventioncan also be used a porous mould, for example as a mould to form glassproducts such as windshield glass.

A set of short metal fibers as of FIG. 3, was used to improve theresistance to thermal cracking and thermal shocks of a ZrO₂—MgO basedceramic glue.

A ceramic material, being a ceramic past, which may be used as ceramicglue, was prepared using 77 gram ZrO₂—MgO based compound and 10 gram ofwater. An amount of a set of short metal fibers having an averageequivalent diameter of 22 μm, of which the length distribution isprovided as indicated with 45 in FIG. 4, is mixed in this ceramic paste,as indicated in Table I.

The ceramic paste was heated to a temperature of 600° C., and thistemperature was kept for 90 sec. after which it was cooled to ambient in60 sec. The number of cracks on an equal surface was counted, and isresumed in Table II.

TABLE II Temperature resistant matrix Set of short % of weight of(ceramic matrix) metal fibers of short metal Number of (gram) 12 μm(gram) fibers (%) cracks (−) 77 0 0 20 77 2 2.5 16 77 4 4.9 8 77 8(sample I) 9.4 0 77 8 (sample II) 9.4 2

An identical result was obtained using a set of short metal fibers of 22μm equivalent diameter.

1. A set of short metal fibers, said fibers having an equivalentdiameter D in the range of 1 to 150 μm, wherein said set of short metalfibers comprises curved fibers and entangled fibers, said curved fibershaving an average length Lc in the range of 10 to 2000 μm, saidentangled fibers having an average length Le, said Le being more than 5times Lc, wherein at least some of said entangled fibers have a majoraxis which changes several times in a non-uniform manner.
 2. A set ofshort metal fibers as in claim 1, wherein lengths of said curved fibersare distributed according to a gamma-distribution.
 3. A set of shortmetal fibers as in claim 1, wherein at least 10% of said short metalfibers are entangled fibers.
 4. A set of short metal fibers as in claim1, L being an average length of fibers in said set of short metalfibers, wherein L/D is larger than
 5. 5. A set of short metal fibers asin claim 1, wherein Lc/D is larger than
 4. 6. A set of short metalfibers as in claim 1, said short metal fibers being stainless steelfibers.
 7. A set of short metal fibers as in claim 1, said set of shortmetal fibers having an apparent density between 10 and 40%.
 8. A set ofshort metal fibers as in claim 1, said curved fibers having a majoraxis, said major axis having a direction, said direction changing morethan 90° for at least 40% of said curved fibers.
 9. A set of short metalfibers as in claim 1, wherein at least some of said entangled fibers areindividually entangled.
 10. A set of short metal fibers as in claim 1,wherein at least some of said entangled fibers are mutually entangled.11. A set of short metal fibers as in claim 1, wherein said entangledfibers are individually or mutually entangled.
 12. A set of short metalfibers as in claim 1, wherein at least some of said entangled fibers arein a shape that resembles at least one of a clew and a pigtail.
 13. Asintered product, comprising short metal fibers as in claim
 1. 14. Asintered product as in claim 13, wherein said product has a porosity ofmore than 60%.
 15. A sintered product as in claim 13, wherein saidproduct has an absolute filter rating between 0.5 and 20 μm.
 16. Asintered product as in claim 13, an absolute filter rating of saidsintered product being between ⅓ and ½ of the equivalent diameter ofsaid short metal fibers.
 17. A method of manufacturing a sinteredproduct according to claim 13, said method comprising the steps of:providing a set of short metal fibers; pouring said set of short metalfibers into a three-dimensional mould or on an essentially planesurface; sintering said set of short metal fibers to form a sinteredproduct according to claim
 13. 18. A method according to claim 17,wherein said method further comprises the step of pressing said shortmetal fibers together before performing said sintering step.
 19. Amethod of filter, comprising: filtering a fluid with the sinteredproduct of claim
 13. 20. A method of filtering, comprising: filteringdiesel exhaust gas with the sintered product of claim
 13. 21. A methodof treating a gas, comprising: removing a substance from a gas with thesintered product of claim 13 carrying a catalyst.
 22. A method ofexchanging heat, comprising: transferring thermal energy from a fluidthat is at a first temperature to the sintered product of claim 13, andlater transferring thermal energy transferred to the sintered product toa fluid that is at a temperature lower than the first temperature.
 23. Amethod of heating a fluid, comprising: exposing a sintered productaccording to claim 13 to a fluid that is heated and transferring thermalenergy from the fluid that is heated to the sintered product; exposingthe sintered product to the fluid after the fluid has cooled, toincrease the temperature of the fluid that has cooled by transferringthermal energy, transferred to the sintered product by the fluid that isheated, from the sintered product to the fluid that has cooled.
 24. Amethod of moulding a substance, comprising: forming a substance in amould, the mould including the sintered product of claim
 13. 25. Amethod of forming glass, comprising: forming glass in a mould, the mouldincluding the sintered product of claim
 13. 26. A Stirling engine,comprising: a heat recuperator including the sintered product of claim13, wherein the sintered product is adapted to exchange heat.
 27. AStirling engine, comprising: a hot chamber; a cold chamber; and asintered product according to claim 13; wherein the Stirling engine isadapted to move a fluid from the hot chamber to the cold chamber whileexposing the sintered product to the fluid to transfer thermal energyfrom the fluid to the sintered product; and wherein the Stirling engineis adapted to move the fluid from the hot chamber to the cold chamberwhile exposing the sintered product to the fluid to transfer thermalenergy, transferred to the sintered product by the fluid, from thesintered product back to the fluid.
 28. A Stirling engine according toclaim 27, further comprising: a passage fluidically between the hotchamber and the cold chamber; wherein the sintered product is mounted inthe passage; and wherein the Stirling engine is adapted to move thefluid through the passage when transferring the fluid from the hotchamber to the cold chamber and when moving the fluid to the coldchamber from the hot chamber.
 29. A ceramic or high temperatureresistant glue, comprising a set of short metal fibers as in claim 1.